Small polymer particles, in the range of 1 to 50 micrometers (.mu.m), have many applications, including light scattering and diffusion, surface coating, intermediates for preparation of chromatographic solid phases, adsorbents, ionexchange materials, and the like. For many such applications, particularly optical and chromatographic ones, uniformity of the particle size is critical to performance. Emulsion polymerization can produce particles up to about 0.5-1.0 .mu.m having a relatively narrow size distribution, but the size is limited by the nature of an emulsion. In such a polymerization the monomers are emulsified in water using a soap, and the polymerization is initiated by a watersoluble, free-radical initiator. The particle size depends more on the composition of the polymerization mixture, i.e., the soap, the stabilizers used and the monomers themselves than on the specific physical conditions of the polymerization.
Suspension polymerization can produce particles from about 50 .mu.m to about 2 mm. The monomer is suspended as droplets in water, and polymerization is initiated by an oil-soluble initiator which partitions into the monomer droplets. Particle size depends on the size of the monomer droplets, which is affected primarily by the stirring rate, the amount of suspension stabilizers, and similar physical conditions for the polymerization. As a result of the inherent variability of these conditions, particle size distribution tends to be broad. Smaller particles, as small as 10 .mu.m, have been reportedly produced by high-shear suspension polymerization, but the size distribution is even broader.
Grinding of polymers that have been made by other processes which produce larger sized polymers, such as bulk polymerization, can lead to particles in the desired size range. Unfortunately, grinding not only results in large size distribution, but causes thermal degradation of heat-sensitive polymers. Thus direct production of particles larger than 1 .mu.m but smaller than 50 .mu.m is difficult.
Multiple-step processes have been used to produce particles within the range of 1-50 .mu.m. In these processes a pre-formed emulsion polymer, or "seed", is used as a starting material. The polymer is swollen, either in an organic solvent or in a monomer which behaves as a swelling solvent for the polymer, and monomer is imbibed into the swollen structure of the polymer and polymerized there, thus increasing the size of, or "growing", the particle. This swelling, imbibing and polymerizing process may be repeated to further increase the size of the particle. Because the emulsion polymerization that forms the seed inherently produces a relatively narrow size distribution, the multiple-step processes have the potential for producing larger particles with similarly narrow size distributions. Not all multiple-step processes live up to this potential, and the swelling and imbibing steps are inherently slow; the reactions may take days. In addition, those processes which involve swelling solvents introduce the economic and environmental problems of solvent recovery and disposal. Special low-shear agitation or highly dilute dispersions may also be required to prevent the particles from coalescing during growth.
Thus it is an object of the present invention to provide a rapid, solvent-free process by which polymer particles in the size range of 1-50 .mu.m and having a narrow size distribution may be made. Other objects of the invention will be apparent from the following description.